Evaluation of Sr2MMoO6 (M 5 Mg, Mn) as anode materials in solid-oxide fuelcells: A neutron diffraction study

L. Troncoso,1,a) M. J. Mart�ınez-Lope,2 J. A. Alonso,2 and M. T. Fern�andez-D�ıaz3

1Departamento de F�ısica, Facultad de Ciencias, Universidad de Santiago de Chile, Av. Lib. BernardoO’Higgins 3363, Santiago, Chile2Instituto de Ciencia de Materiales, CSIC, Cantoblanco, 28049 Madrid, Spain3Institut Laue-Langevin, B.P. 156, F-38042 Grenoble Cedex 9, France

(Received 23 May 2012; accepted 17 December 2012; published online 10 January 2013)

Sr2MMoO6 (M¼Mg, Mn) double perovskites have recently been proposed as anode materials in

solid-oxide fuel cells (SOFC). The evolution of their crystal structures has been followed by “in situ”

temperature-dependent neutron powder diffraction from 25 �C room temperature (RT) to 930 �C by

heating in ultrahigh vacuum (PO2� 10�6 Torr) in order to simulate the reducing atmosphere

corresponding to the working conditions of an anode in a SOFC. At RT, the samples are described as

tetragonal (I4/m space group) and monoclinic (P21/n) for M¼Mg, Mn, respectively. Sr2MgMoO6

undergoes a structural phase transition from tetragonal to cubic (Fm-3m) below 300 �C; Sr2MnMoO6

experiences two consecutive phase transitions to tetragonal (I4/m) and finally cubic (Fm-3m) at

600 �C and above. In the cubic phases, the absence of octahedral tilting accounts for a good overlap

between the oxygen and transition-metal orbitals, resulting in a good electronic conductivity; a high

mobility of the oxygen atoms is derived from the elevated displacement parameters, for instance

3.0 A2 and 4.6 A2 at 930 �C for M¼Mg, Mn, respectively. Both factors contribute to the excellent

performance described for these mixed ionic and electronic conductor oxides as anodes in single fuel

cells. From dilatometric measurements, the thermal expansion coefficients (TEC) in the cubic region

are 12.7� 10�6 K�1 and 13.0� 10�6 K�1 for M¼Mg and Mn, respectively. These figures are

comparable to those obtained from the mentioned structural analysis; moreover, the TECs for the

cubic phases perfectly match those of the usual electrolytes in a SOFC. VC 2013 American Institute ofPhysics. [http://dx.doi.org/10.1063/1.4774764]

INTRODUCTION

Many efforts are being devoted to design and prepare

anode materials for solid-oxide fuel cells (SOFC) able to per-

form at intermediate temperatures (550-850 �C), motivated

by the purpose of decreasing the operation cost. A necessary

requirement for a material to perform well as an SOFC anode

in such conditions is that it is a “mixed ionic and electronic

conductor” (MIEC). It must also have a thermal-expansion

coefficient that is compatible with that of the solid oxide

electrolyte. ABO3 perovskites exhibit a well-known ability

to accommodate a wide range of oxygen deficiency, giving

good oxide ion conduction and mixed oxidation states of

B-site transition metal, resulting in good electronic conduc-

tion.1,2 Double perovskite A2BB’O6 oxides contain two dif-

ferent B and B0 cations, exhibiting 1:1 long range ordering;

typically A are alkaline-earth or rare-earth ions and B and B0

transition metal ions. The crystallographic structure of these

oxides can be cubic, orthorhombic, or monoclinic, depending

on the occurrence of octahedral tilting.3 Double perovskites

have been proposed as good anode materials in SOFC with

better fuel flexibility than the usual metal-electrolyte

cermets.4 In particular, the double-perovskite oxides

Sr2MMoO6 (M¼Mg, Mn) have been recently shown to pro-

vide a stable performance as anode of a SOFC operating on

H2 as fuel and a promising performance with CH4 as fuel.5

In the present work, we are interested in evaluating

the structural reasons for such a good performance. We

have investigated the thermal evolution of the crystal

structure of Sr2MMoO6 (M¼Mg, Mn) in the actual work-

ing conditions of an anode in a SOFC from “in situ” neu-

tron powder diffraction (NPD) experiments, in complement

with thermal expansion measurements under reducing

atmosphere.

FIG. 1. XRD patterns of Sr2MMoO6 (M¼Mg, Mn).

a)Author to whom correspondence should be addressed. Electronic mail:

Loreto.troncoso@usach.cl.

0021-8979/2013/113(2)/023511/8/$30.00 VC 2013 American Institute of Physics113, 023511-1

JOURNAL OF APPLIED PHYSICS 113, 023511 (2013)

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EXPERIMENTAL

The stoichiometric Sr2MMoO6 (M¼Mg, Mn) oxides

were prepared via a nitrate-citrate route. The starting materi-

als Sr(NO3)2, (NH4)6Mo7O24�4H2O, and MgO or MnCO3 of

analytical grade were dissolved under stirring in 250 ml of

10% citric-acid aqueous with several droplets of concen-

trated HNO3. The mixture of citrate and nitrate solutions was

slowly evaporated, leading to organic resins where a random

distribution of the involved cations is expected at an atomic

level. The resin was dried at 120 �C and slowly decomposed

at temperatures up to 600 �C. A subsequent treatment at

800 �C for 2 h ensures the total elimination of all the organic

materials and nitrates. For M¼Mn, the resulting highly re-

active precursors were heated at 1000 �C for 12 h in air fol-

lowed by a reducing treatment at 1000 �C for 8 h in a 15%

H2-85% N2 flow. For M¼Mg, the precursor powders were

heated at 1150 �C in air followed by a thermal treatment at

1200 �C for 12 h in a 5% H2-95% N2 flow.

The reaction products were characterized by x-ray dif-

fraction (XRD) for phase identification and to asses phase

purity. The characterization was performed with a Bruker-

axs D8 diffractometer (40 kV, 30 mA) in Bragg-Brentano

reflection geometry with Cu Ka1 (k¼ 1.5406 A) and Cu Ka2

(k¼ 1.5444 A) radiation. NPD data were collected in the

D2B diffractometer at the Institut Laue-Langevin, Grenoble,

in the high-resolution configuration with a neutron wave-

length k¼ 1.549 A. About 2 g of the sample was contained in

a vanadium can and placed in the isothermal zone of a fur-

nace with a vanadium resistor operating under vacuum (PO2

� 10�6 Torr). The measurements were carried out at 25, 300,

FIG. 2. Observed (crosses), calculated (full line), and difference (at the bot-

tom) NPD profiles for Sr2MgMoO6-d at (a) 25 �C refined in the tetragonal

I4/m space group and (b) 930 �C, refined in the cubic Fm-3m space group.

The vertical markers correspond to the allowed Bragg reflections.

TABLE I. Structural parameters for Sr2MgMoO6 from in situ NPD data at different temperatures. The bold characters indicate the symmetry equivalent positions.

T (�C) 25 300 600 800 930

Space group I 4/m Fm-3m Fm-3m Fm-3m Fm-3m

a (A) 5.5712(1) 7.9252(1) 7.9569(1) 7.9788(1) 7.9936(2)

b (A) 5.5712(1) a a a a

c (A) 7.9234(2) a a a a

V (A3) 245.93 (0.8) 497.77(1) 503.76(2) 507.94(2) 510.77(2)

Sr 4d (0 1/2 1/4) 8c (1/4,1/4,1/4) 8c (1/4,1/4,1/4) 8c (1/4,1/4,1/4) 8c (1/4,1/4,1/4)

Biso (A2) 0.47(5) 1.37 2.00 2.45 2.73

Mg 2a (0 0 0) 4a (0 0 0) 4a (0 0 0) 4a (0 0 0) 4a (0 0 0)

Biso (A2) 0.24(4) 0.80(3) 1.25(2) 1.37(2) 1.70(3)

Focc (Mg/Mo) 0.87(7)/0.13(7) 0.79(7)/0.21(7) 0.91(6)/0.09(6) 0.84(7)/0.16(7)

Mo 2b (0 0 1/2) 4b(1/2,0,0) 4b(1/2,0,0) 4b(1/2,0,0) 4b(1/2,0,0)

Biso (A2) 0.16(3) 0.50(2) 0.72(2) 1.00(2) 1.17(2)

Focc Mo/Mg 0.13(7)/0.87(7) 0.21(7)/0.79(7) 0.13(6)/0.87(6) 0.09(7)/0.91(7)

O1 4e (0 0 z) 24e (x,0,0) 24e (x,0,0) 24e (x,0,0) 24e (x,0,0)

x 0 0.2594(2) 0.2599(2) 0.2603(3) 0.2605(3)

z 0.2595(8) 0 0 0 0

Beq (A2) 0.49 1.68 2.27 2.67 3.02

O2 8h (x y 0)

x 0.2830(7) … … … …

y 0.2303(6) … … … …

Beq (A2) 0.6710

Reliability factors

v2 2.07 2.02 1.87 1.51 1.56

Rp (%) 3.92 3.40 3.45 3.16 3.22

Rwp (%) 5.02 4.42 4.47 4.04 4.10

RBragg (%) 4.86 2.42 2.54 2.58 2.98

023511-2 Troncoso et al. J. Appl. Phys. 113, 023511 (2013)

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600, 800, and 930 �C for M¼Mg; and 25, 200, 400, 600,

800, and 930 �C for M¼Mn. The counting time for each pat-

tern was 2 h. The diffraction data were analysed by the Riet-

veld method6 with the FULLPROF program.7 The line shape of

the diffraction peaks was generated by a pseudo-Voigt func-

tion. The following parameters were refined: background

points, zero shift, half width, pseudo-Voigt, scale factor, and

unit-cell parameters. Positional factors for oxygen atoms and

anisotropic displacement factors were also refined for the

NPD data. The coherent scattering lengths for Sr, Mg, Mn,

Mo, and O were 7.02, 2.49, �3.73, 6.715, and 5.803 fm,

respectively.

For the thermal expansion measurements of Sr2MMoO6

(M¼Mg, Mn), the powders were uniaxially pressed (�1

ton) into a cylindrical pellet sintered at 1000 �C for 6 h in 5%

H2/95% N2 atmosphere and at 1000 �C for 6 h in 15% H2/

85% N2, respectively. The thermal expansion was deter-

mined by dilatometric analysis using a Linseis L75H1000

dilatometer under a 5% H2/N2 flow. Both pellet were densi-

fied up to 90%–95% and polished to obtain a thickness as

uniform as possible. The densities of the pellets were deter-

mined using the following equations:

u ¼ m

V(1)

and

ux ¼Z �W

V � Navog; (2)

where u and ux are the experimental and theoretical den-

sities of the sample, respectively.

The percentage of the degree of densification for the pel-

lets is obtained as (u/ux)� 100.

RESULTS AND DISCUSSION

Sr2MMoO6 (M¼Mg, Mn) were obtained as black,

well-crystallized powders. No impurity phases were

detected. The laboratory XRD diagrams at room temperature

(RT) of both perovskites are shown in Fig. 1.

TABLE II. Anisotropic factors for Sr2MgMoO6 from in situ NPD data at

different temperatures.

T (�C) 25 300 600 800 930

O1

b11 0.0054(6) 0.0019(2) 0.0027(3) 0.0038(3) 0.0046(3)

b22 0.0054(6) 0.0091(2) 0.0121(2) 0.0138(2) 0.0154(2)

b33 0.0006(5) 0.0091(2) 0.0121(2) 0.0138(2) 0.0154(2)

O2

b11 0.0064(14)

b22 0.0016(15)

b33 0.0040(3)

TABLE III. Interatomic distances (A) and angles (�) for tetragonal and cubic Sr2MgMoO6 at increasing temperatures. The bold characters indicate the average

atomic differences.

T (�C) 25 T (�C) 300 600 800 930

Mg–O1 (x2) 2.056(6) Mg–O1 (x12) 2.056(2) 2.068(2) 2.076(2) 2.082(2)

Mg–O2 (x4) 2.033(4)

hMg–Oi 2.045(5) hMg–Oi 2.056(2) 2.068(2) 2.076(2) 2.082(2)

Mo–O1 (x2) 1.906(6) Mo–O1 (x6) 1.907(2) 1.910(2) 1.913(2) 1.914(2)

Mo–O2 (x4) 1.929(4)

hMo–Oi 1.918(5) hMo–Oi 1.907(2) 1.910(2) 1.913(2) 1.914(2)

Sr–O1 (x4) 2.787(2) Sr–O1 (x6) 2.803(5) 2.814(6) 2.822(8) 2.827(8)

Sr–O2 (x4) 2.944(3)

Sr–O2 (x4) 2.652(2)

hSr–Oi 2.794(3) hSr–Oi 2.803(5) 2.814(6) 2.822(8) 2.827(8)

Mg–O1–Mo 180(3) Mg–O1–Mo 180.0 180.0 180.0 180.0

Mg–O2–Mo 167.96(15)

FIG. 3. Schematic representation of the crystal structure of (a) tetragonal

Sr2MMoO6, approximately projected along the [001] direction, showing the

antiphase tilting of the MO6 and MoO6 octahedra; (b) cubic phase, showing

the untilted, alternating MO6 and MoO6 octahedra; (c) monoclinic structure

of Sr2MnMoO6, highlighting the tilting of the MnO6 and MoO6 octahedra.

023511-3 Troncoso et al. J. Appl. Phys. 113, 023511 (2013)

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Sr2MgMoO6

At RT, the crystal structure was refined from NPD data

as a tetragonal double perovskite defined in the I4/m group

(No. 87) with a¼ 5.5712(1) A and c¼ 7.9234(2) A, in agree-

ment with previous reports.8 For the tetragonal structure, Sr

atoms were located at 4d positions, Mg at 2a, Mo at 2b, and

oxygen atoms at 4e and 8h sites. Fig. 2(a) shows the quality

of the fit at RT. Table I contains the main atomic parameters

after the refinement, Table II lists the anisotropic thermal

factors and Table III lists the main interatomic distances and

angles. A view of the tetragonal crystal structure is shown in

Fig. 3(a). It contains alternating MgO6 and MoO6 octahedra,

tilted in anti-phase by 6.05�. This corresponds to the a0a0c�

tilting system (Glazer’s notation) derived by Woodward for

1:1 ordering of double perovskites,9 consistent with space

group I4/m.

A structural transition from tetragonal to cubic symme-

try is observed, occurring between 25 and 300 �C. In the

300–930 �C temperature range, the crystal structure for

Sr2MgMoO6 was successfully refined in the Fm-3m space

group (No. 225). This is a double perovskite structure char-

acterized by the long-range 1:1 ordering the Mg and Mo

over the B sites of the perovskite structure, with doubled

unit-cell parameter with respect to the arystotype a0, as

a¼ 2xa0. In this structure, Sr atoms are located at the 8cpositions, Mg and Mo are placed at the 4a and 4b sites,

respectively, and the oxygen atoms occupy the 24e positions.

The antisite disordering, assuming that some Mg occupies

the Mo sites and vice-versa was also refined. The refinement

of the oxygen occupancy factors did not lead to a significant

oxygen deficiency within the standard deviations in all the

temperature range. Fig. 2(b) illustrates the goodness of the fit

TABLE IV. Structural parameters for Sr2MnMoO6 from in situ NPD data at different temperatures. The bold characters indicate the symmetry equivalent positions.

T (�C) 25 200 400 600 800 930

Space group P21/n P21/n I4/m Fm-3m Fm-3m Fm-3m

a (A) 5.6671(2) 5.6859(5) 5.6790(3) 8.0610(2) 8.0821(3) 8.0954(3)

b (A) 5.6538(1) 5.6642(4) 5.6790(3) a a A

c (A) 7.9968(2) 8.0104(6) 8.0602(6) a a A

a (�) 90 90 90 90 90 90

b (�) 89.93(3) 89.96(4) a a a ac (�) a 90 a a a aVolume (A3) 256.22(1) 257.99(4) 259.96(3) 523.80(3) 527.93(3) 530.54(3)

Atoms

Sr 4e (x y z) 4e (x y z) 4d (0 1/2 1/4) 8c (1/4 1/4 1/4) 8c (1/4 1/4 1/4) 8c (1/4 1/4 1/4)

x 0.997(1) 1.026(4) 0 0.25 0.25 0.25

y 0.019(1) 0.002 (3) 0.5 x x X

z 0.249(1) 0.248(2) 0.25 x x X

Beq (A2) 1.10 1.88 2.29(2) 2.98 3.37 3.80

Mn 2d (1/2 0 0) 2d (1/2 0 0) 2a (0 0 0) 4a (0 0 0) 4a (0 0 0) 4a (0 0 0)

Focc (Mn/Mo) 0.96(5)/0.04(5) 0.96(5)/0.04(5) 0.96(5)/0.04(5)

Beq (A2) 1.24 0.86 1.30(5) 1.72(3) 1.71(3) 2.09(3)

Mo 2c (1/2 0 1/2) 2c (1/2 0 1/2) 2b (0 0 1/2) 4b (1/2 0 0) 4b (1/2 0 0) 4b (1/2 0 0)

Focc (Mo/Mn) 0.04(5)/0.96(5) 0.04(5)/0.96(5) 0.04(5)/0.96(5)

Beq (A2) 0.51 0.73 1.17(2) 1.37(2) 1.50(1) 1.56(2)

O1 4e (x y z) 4e (x y z) 4e (0 0 Z) 24e (x 0 0) 24e (x 0 0) 24e (x 0 0)

x 0.054(1) 0.031(5) 0 0.264(3) 0.264(3) 0.264(3)

y 0.493(1) 0.499(4) 0 0 0 0

z 0.237(1) 0.235(2) 0.264(1) 0 0 0

Beq (A2) 0.64 3.19 3.70 4.14 4.53 4.63

O2 4e (x y z) 4e (x y z) 8h (x y 0)

x 0.737(1) 0.732(3) 0.289(1)

y 0.290(1) 0.268(3) 0.238(1)

z 0.028(1) 0.019(2) 0.000(1)

Beq (A2) 1.08 4.22 3.36

O3 4e (x y z) 4e (x y z)

x 0.207(1) 0.222(2)

y 0.234(1) 0.252(3)

z 0.973(1) 0.969(2)

Beq (A2) 0.94 2.89

Reliability factors

v2 1.33 1.17 1.28 1.32 1.32 1.51

Rp (%) 3.91 2.77 2.85 2.90 2.92 2.97

Rwp (%) 4.99 3.49 3.61 3.67 3.66 3.85

RBragg (%) 4.15 3.91 3.46 3.74 3.56 4.25

023511-4 Troncoso et al. J. Appl. Phys. 113, 023511 (2013)

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of the cubic phase at 930 �C for this double perovskite, and

Fig. 3(b) displays a schematic view of the cubic 2a0 struc-

ture, showing the untilted, alternating MgO6 and MoO6

octahedra.

Sr2MnMoO6

At RT, the crystal structure has been Rietveld-refined in

the monoclinic P21/n space group (No. 14) with

a¼ 5.6671(1) A, b¼ 5.6537(1) A, and c¼ 7.9969(2) A, in

agreement with previous reports.3 In this model, Sr, O1, O2,

and O3 atoms are located at 4e positions, Mn at 2d and Mo

at 2c sites. Table IV lists the main atomic parameters after

the refinement and Table V the anisotropic displacement fac-

tors. Fig. 4(a) shows the quality of the fit at RT. A view of

the monoclinic crystal structure is shown in Fig. 3(c). It con-

tains alternating MnO6 and MoO6 octahedral, tilted by 8.9�,8.8�, and 9.3� along the three pseudocubic axis, according to

the tilting scheme a�bþc� as derived by Woodward for

long-range ordered P21/n double perovskites.9

At 200 �C, the structure remains monoclinic. Between

200 and 400 �C, there is a phase transition to a tetragonal

structure, with the appearance of a characteristic splitting of

some high-angle peaks, as shown in the inset of Fig. 4(a). At

400 �C, the crystal structure can be refined in the same tet-

ragonal model described in the I4/m space group for

M¼Mg. The MnO6 and MoO6 octahedra are tilted in anti-

phase by 5.8�. Finally, at 600 �C, the mentioned splitting dis-

appears as the sample becomes cubic (space group Fm-3m),

as shown in the inset of Fig. 4(b); the 800 and 930 �C data

were also fitted in the cubic structure. As observed for

M¼Mg, there is no deviation from the full oxygen stoichi-

ometry in all the temperature range. Figures 4(a) and 4(b)

illustrate the quality of the refinement. Table VI summarizes

the main interatomic distances and angles from RT to 930 �Cof the perovskite structure.

The equivalent isotropic displacement factors of the ox-

ygen atoms in the cubic phase increase for both structures,

suggesting an elevated ionic mobility of oxide ions espe-

cially at high temperatures T> 300 �C (Fig. 5).

Thermal expansion

From the thermal variation of the unit-cell parameters,

we have estimated the thermal expansion coefficients (TEC)

using the following equation:

TEC ¼ ðaTf-aTi=aTiÞ=Tf- Ti: (3)

In the high-temperature region where the cubic Fm-3mphase is stable, a TEC of 13.5� 10�6 K�1 was obtained

between Ti¼ 300 �C and Tf¼ 930 �C for Sr2MgMoO6. For

Sr2MnMoO6, the TEC obtained was 12.8� 10�6 K�1

between Ti¼ 600 �C and Tf¼ 930 �C.

Alternatively, the TECs were determined by dilatomet-

ric analysis. The measurement of TECs was carried out on

densified samples to avoid the contraction effect that would

be observed upon the densification process at high tempera-

tures. Both samples were cycled between 35 and 900 �C for

several times; the data were only recorded during the heating

runs. Fig. 6 shows no abrupt changes in thermal expansion;

the tetragonal to cubic phase transition is probably too subtle

to give rise to a significant change of slope in the corre-

sponding temperature range. Therefore, the absence of ab-

rupt changes makes both materials suitable as electrodes in

SOFC, from this point of view. The average TEC obtained

from both curves are 12.7� 10�6 K�1 and 13.0� 10�6 K�1

for M¼Mg and Mn, respectively, in good agreement with

TABLE V. Anisotropic factors for Sr2MnMoO6 from in situ NPD data at different temperatures.

T (�C) RT 200 T(�C) RT 200 400 600 800 930

Sr O1

b11 0.006(1) 0.009(2) b11 0.009(2) 0.055(1) 0.037(3) 0.005(5) 0.006 (5) 0.007(5)

b22 0.010(1) 0.020(4) b22 0.005(2) 0.019(6) 0.037(3) 0.022(4) 0.023(4) 0.023(4)

b33 0.005(1) 0.007(4) b33 0.001(1) 0.001(2) 0.007(1) 0.022(4) 0.023(4) 0.023(4)

b12 �0.002(1) �0.001(3) b12 0.000(1) 0.003(7)

b13 �0.000(1) 0.001(3) b13 �0.001(9) �0.005(3)

b23 0.001(1) 0.003(3) b23 0.001(1) 0.009(3)

Mn O2

b11 0.010(5) 0.001(5) b11 0.006(1) 0.064(8) 0.021(2)

b22 0.003(4) 0.006(6) b22 0.007(1) 0.018(6) 0.015(3)

b33 0.008(3) 0.006(4) b33 0.006(1) 0.008(3) 0.021(1)

b12 0.000(1) 0.001(6) b12 �0.002(1) 0.011(6) �0.009(2)

b13 �0.007(4) �0.004(7) b13 0.001(8) 0.007(4) 0.000(1)

b23 �0.001(2) 0.008(5) b23 0.003(1) 0.007(5) 0.000(1)

Mo O3

b11 0.003(2) 0.006(3) b11 0.009(1) 0.013(3)

b22 0.005(2) 0.005(3) b22 0.010(2) 0.037(7)

b33 0.002(1) 0.003(2) b12 0.002(1) 0.009(3)

b12 �0.002(2) 0.001(3) b12 0.005(2) 0.023(4)

b13 0.002(2) �0.005(5) b13 �0.002(1) 0.001(2)

b23 0.002(1) �0.002(2) b23 �0.002(1) 0.002(5)

023511-5 Troncoso et al. J. Appl. Phys. 113, 023511 (2013)

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those determined from NPD data. The thermal expansion

coefficients of the anode must be compatible with that of the

solid electrolyte in order to prevent the fracture of the SOFC

during fuel cell operation cycles.10 Typical values for the

electrolyte are between 11.4� 10�6 and 12.1� 10�6

K�1,11,12 similar to those observed for the present materials.

A last issue is that concerning the full oxygen stoichiom-

etry determined by neutron diffraction for both perovskite

oxides. In principle, it is believed that the ionic transport,

required for the performance of the anode materials, works

with a vacancy mechanism, the pre-existing vacancies being

filled by neighboring oxygen atoms. However, in this case

the neutron data consistently indicate, for both compounds at

all the measured temperatures, that the perovskites are

strictly oxygen stoichiometric. This behavior has been

observed before in other double perovskite oxides, also used

as anodes in SOFC.13 As it is certain that all these materials

combine an excellent electronic conductivity with a suffi-

cient ionic transport of oxide anions, we could imagine a

mechanism where surface oxygen atoms in contact with the

fuel (typically H2) are removed to give H2O, and the created

vacancies (in the surface) are instantaneously filled by inter-

nal oxygen, the vacancies diffusing across the bulk, finally

reaching the electrolyte where they are recombined with O2�

ions. In other words, these double perovskites do not contain

a measurable concentration of permanent vacancies, but the

lability of the oxygen atoms is sufficiently high to ensure an

ionic transport from the surface to the electrolyte, via tran-

sient vacancies. Such a mechanism is possible in the present

Mo-containing perovskites since, as noted by Goodenough

et al.,14 the ability of Mo to form molybdyl ions allows a

six-fold coordinated Mo(VI) to accept an electron while los-

ing an oxide ligand and, on the other hand, the partner

octahedral-site cations (Mn and Mg) are each stable in less

FIG. 4. Observed (crosses), calculated (full line), and difference (at the bot-

tom) NPD profiles for Sr2MnMoO6 at (a) 25 �C, refined in P21/n, (b) 930 �Crefined in the cubic Fm-3m space group. The vertical markers correspond to

the allowed Bragg reflections. The insets show high-angle regions highlight-

ing the evolution of some superstructure reflection.

TABLE VI. Interatomic distances (A) and angles (�) for Sr2MnMoO6 from in situ NPD data at increasing temperatures. The bold characters indicate the aver-

age atomic differences.

T (�C) 25 200 T (�C) 400 T (�C) 600 800 930

Mn–O1 (x2) 2.131(4) 2.130(1) Mn–O1 (x2) 2.126(8) Mn–O1 (x12) 2.127(2) 2.130(2) 2.133(2)

Mn–O2 (x2) 2.125(4) 2.017(2) Mn–O2 (x4) 2.124(7)

Mn–O3 (x2) 2.131(4) 2.144(1)

hMn–Oi 2.129(4) 2.097(2) hMn–Oi 2.125(8) hMn–Oi 2.127(2) 2.130(2) 2.133(2)

Mo–O1 (x2) 1.916(4) 1.891(1) Mo–O1 (x2) 1.904(8) Mo–O1 (x6) 1.903(2) 1.911(2) 1.915(2)

Mo–O2 (x2) 1.924(4) 2.018(1) Mo–O2 (x4) 1.912(6)

Mo–O3 (x2) 1.924(4) 1.904(2)

hMo–Oi 1.921(4) 1.937(2) hMo–Oi 1.908(7) hMo–Oi 1.903(2) 1.911(2) 1.915(2)

Sr–O1 2.982(4) 2.85(3) Sr–O1 (x3) 2.842(3) Sr–O1 (x6) 2.852(1) 2.859(1) 2.864(2)

Sr–O1 2.714(4) 2.82(3)

Sr–O1 2.551(5) 3.10(4)

Sr–O2 2.770(7) 2.86(2) Sr–O2 (x4) 2.994 (5)

Sr–O2 2.560(7) 2.68(2) Sr–O2 (x4) 2.707 (4)

Sr–O2 2.829(8) 3.00(2)

Sr–O3 2.793(8) 2.89(2)

Sr–O3 2.803(7) 3.07(2)

Sr–O3 2.551(7) 2.62(2)

hSr–Oi 2.728(6) 2.578(3) hSr–Oi 2.851(4) hSr–Oi 2.852(1) 2.859(1) 2.864(2)

Mn–O1–Mo 162.3(2) 169.9(5) Mn–O1–Mo 180.0 Mn–O1–Mo 180.0 180.0 180.0

Mn–O2–Mo 162.4(2) 168.1(6) Mn–O2–Mo 168.4(3)

Mn–O3–Mo 161.5(2) 164.8(6)

023511-6 Troncoso et al. J. Appl. Phys. 113, 023511 (2013)

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than six-fold oxygen coordination, thus accepting the fast

creation and displacement of oxygen vacancies.

CONCLUSIONS

Sr2MMoO6 (M¼Mg, Mn) double perovskites have

recently been described as good candidates for anode materi-

als in SOFCs single cells.15,16 In the present paper, we have

investigated the thermal evolution of their crystal structures

that accounts for the observed performance, by in situ neu-

tron powder diffraction in the usual working conditions of an

anode in SOFCs, in complement with dilatometric analysis.

A phase transition of the RT tetragonal I4/m structure to a

cubic Fm-3m has been identified below 300 �C for M¼Mg,

whereas for M¼Mn there are two successive phase transi-

tions in the investigated temperature range, from the mono-

clinic P21/n structure stable at RT to tetragonal I4/m (below

400 �C) and finally to cubic Fm-3m (below 600 �C). There is

a progressive reduction of the tilting system, from a�b�cþ

(P21/n) to a0a0c� (I4/m) to a0a0a0 (Fm-3m). In the high-

temperature cubic phase, present at the working temperature

of a SOFC, above 600 �C, the absence of tilting of the MO6

(M¼Mg, Mn) and MoO6 octahedra favours the orbital over-

lap and the electronic conductivity; the lability and high

mobility of the oxygen atoms derived from the elevated

displacement factors in this MIEC oxide account for the

excellent performance of these materials as anodes recently

described in single fuel cells. The moderated TEC values ob-

tained by dilatometry in the cubic region (12.7� 10�6 K�1

and 13.0� 10�6 K�1, respectively) are comparable to the

values obtained from the refinement of the neutron diffrac-

tion data in the heating run and perfectly match those of the

usual electrolytes in a SOFC.

ACKNOWLEDGMENTS

We are grateful to Professor Serafini for a critical read-

ing of the manuscript. We thank ILL for making the beam-

time available. We acknowledge the financial support of the

Spanish “Ministerio de Ciencia e Innovaci�on” (MICINN) to

the project MAT2010-16404. L.T. thanks the financial sup-

port of CONICYT for “Beca Nacional de Doctorado 2009.”

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